In the rapid development nowadays of optical fibers and fiber components in the world more and more complicated optical fiber systems are utilized for use in the technical fields of communication and sensing. In some such advanced systems it would be advantageous if a fusioned fiber splice could be made in situ to be built into an optical fiber line and if the splice could be given a predetermined lateral offset of the fiber end claddings or of the cores of the spliced optical fiber ends. Such an offset in the splice is necessary for e.g. splicing optical fibers having strongly eccentrically located fiber cores with an alignment of the cores for giving a splice having a low attenuation, for producing in situ, from optical fibers of the standard type used for communication, attenuation elements having a predetermined, small reflective capability, splicing an optical D-fiber to a conventional cylindrical optical fiber, splicing conventional optical fibers having a single core to optical fibers having double cores, etc.
There are many methods of producing a fusioned or welded splice of two optical fibers such that in the splice there is a lateral offset, as seen perpendicularly to the longitudinal direction of the fibers, of the exterior surfaces of the claddings of the fibers. However, there exists a significant difficulty in controlling or regulating the offset, so that it achieves a desired value with a good accuracy, since the surface tension effect during the melted condition in the splicing region, which exists during the fusion welding, tends to reduce every lateral offset between the exterior surfaces of the fiber claddings. The tensile velocity, which acts in the transverse direction of the fibers and is caused by the surface tension, is a function of temperature of the material in the splicing region and of the lateral offset of the fiber claddings, so that for instance for a very small offset the tensile velocity is very small or even quite negligible. Further, since the temperature in the splicing region depends on various exterior conditions such as, for conventional melt-fusioning by means of an electric arc generated between two welding electrodes, the state of the electrodes, the current through the electrodes during the melt-fusioning, the pressure of the ambient air, etc., it is impossible to maintain the temperature constant from one welding occasion to another. Thereby the lateral offset of the fiber claddings after the fusion welding will also vary, although the same time and current are used for the fusion welding. Further, an additional effect is that the larger the offset of the fiber claddings, the larger is the above mentioned restoring velocity or the velocity, with which the surface tension tries to reduce the offset of the exterior surfaces of the fiber claddings to a value near zero.
In order to control the offset of the fiber cores during the welding of two optical fibers of conventional type a real-time-control based on image processing of a picture taken thereof in the heated fiber ends was developed, see the Swedish Patent Application No. 9201235-0, filed Apr. 16, 1992. In order that it will be possible to use such a picture and in particular that it will be possible to distinguish therein the core/cores of a fiber or fibers, the heating temperature, which for electric arc welding corresponds to or depends on the current through the electrodes, must have a sufficiently high value, so that for instance for quartz standard fibers of type single mode widely used for communication and having cladding diameters of 125 .mu.m the welding current must be larger than 13 mA. This method can work well when the desired offset of the fiber cores is smaller than 2 .mu.m, since then the restoring tensile velocity in the transverse direction of the fibers is very low. However, the larger the offset of the fiber cores, the larger is the restoring tensile velocity depending on the surface tension and this effect is in particular observable during the rather high currents, which are required for taking such a picture of a heated fiber or of heated fiber ends. When the offset of the fiber claddings is larger than 2 .mu.m and when a welding current, which is sufficiently large in order that such a warm fiber picture should be obtained, is applied during the short heating times, of about 0.3 seconds, which are used according to the previous method mentioned above, a large reduction of the lateral offset of the exterior surface of the fiber claddings is obtained, in the normal case more than 0.8 .mu.m for each such period of pulsed heating. This reduction can even exceed 1 .mu.m for an initial lateral offset of the exterior surfaces of the fiber claddings comprising 8 .mu.m. It makes the accuracy of the final offset of the fiber claddings very low in this case, i.e. the resolution of our previous real-time-control based only on pictures obtained of heated fiber ends can be so bad as 0.8 .mu.m to 1.0 .mu.m, when the lateral offset between the exterior surfaces of the fiber end claddings is larger than 2.0 .mu.m. This resolution of the real-time-control can generally be defined as the distance, by which the lateral offset of the exterior surfaces of the fiber claddings is changed, during the time period, which is required for taking and analyzing a picture and which can have the magnitude of order 0.3 to 0.5 seconds.
The picture, which is used in the previously developed method, is, as has been already indicated, a warm fiber picture, i.e. a picture of a fiber, which is not illuminated by any light source from the exterior but only emits light depending on the rather high temperature, to which it is heated, in the standard case by an electric arc. In order to obtain such a usable warm fiber picture, where material inhomogeneities and in particular the fiber cores are distinguishable, the heating temperature must be rather high, as has been mentioned already. The picture which is obtained of a fiber or of fiber ends in a cold state, here called a cold fiber picture, is a picture of a fiber, which is illuminated by means of some exterior light source from a side of the fiber, generally perpendicularly to the longitudinal direction thereof, and is observed on the opposite side of the fiber. In such a picture, generally the fiber cores are not distinguishable, unless special, costly constructed optical systems having for instance highly resolving lenses are used.
Relevant prior art within the fiber splicing technique appears from among other sources the Swedish Patent Applications Nos. SE-A 9100979-5, 9100978-7, 9201817-5 and 9201818-3. Pulsed heating of splices is used in the method disclosed in JP-A 2-6908 where the external diameter and axial shifts of five parallel fiber splices are monitored.